There was a strange lack of any bestseller list update in last... two weeks, I presume? Maybe more.

I wonder if that means it gets updated monthly now, or is it plain broken.

It is/was updated at the end of the month.
http://www.gog.com/en/forum/general/tyrian_200o_is_the_las_game_on_the_bestselling_list

When Planescape Torment was released, it was updated daily, I was checking every day to see how fast it climbs up.

For Witcher 1 it was also updated every one or two days.

I'm talking about bestselling on the catalogue page, not on the main page.

Maybe these were important titles from PR point of view so they updated list daily?

Guess what, it did update today :).

Yeah, but it's strange that Dungeon Keeper and Crusader are not higher.

Perhaps it's a new kind of broken :).

I thought the best-sellling filter in the catalogue updates weekly myself around weekends. That's what I noticed anyway while browsing.

Maybe they changed it during spring update of the website.

By the way, have you noticed how many rpgs are in the top 20 bestselling games? I'm excluding those games that are or were free (Jagged Alliance, Tex Murphy)